1. Field of the Invention
The present invention relates to a driving device using a piezoelectric actuator.
2. Description of the Related Art
Positioning devices currently used to position optical elements in semiconductor exposure apparatuses are required to have a high positioning accuracy of the order of nanometers. For this reason, piezoelectric actuators are used to drive the positioning devices.
A laminated piezoelectric actuator has a structure in which thin layers of piezoelectric ceramics and electrode layers are alternately stacked to form several hundreds of layers. In general, polarization and voltage application are performed by using a wire connected to an electrode layer serving as an anode and a wire connected to an electrode layer serving as a cathode.
Since the voltage is applied via a pair of wires in the laminated piezoelectric actuator, insulation performance may be reduced by migration, and this may cause a short circuit between the electrodes. Also, failure may occur in wire connection. In this case, it is difficult to apply the voltage to the piezoelectric ceramics, and the actuator may lose its function.
The following mechanisms for avoiding driving failure of the laminated piezoelectric actuator have been disclosed.
Japanese Patent Laid-Open No. 2002-257000 discloses that driving can be performed even when failure occurs in wire connection in a fuel injection apparatus including a plurality of piezoelectric actuators. More specifically, a plurality of piezoelectric actuators and a plurality of switches are connected in one-to-one correspondence, and, when failure occurs in wire connection for one of the piezoelectric actuators, the corresponding switch is merely turned off, so that the driving of the remaining piezoelectric actuators is prevented from being affected by the failure.
Japanese Patent Laid-Open No. 2002-185056 discloses a technique of checking a piezoelectric actuator for failure. In this technique, a reference piezoelectric actuator that normally operates is connected to a target piezoelectric actuator in parallel. First, charges accumulated by charging the reference piezoelectric actuator at a fixed voltage are released to the target piezoelectric actuator. Then, the voltage of the reference piezoelectric actuator is detected so as to check whether the target piezoelectric actuator is normal.
Unfortunately, the above-described known techniques are not suitable for precise positioning apparatuses like exposure apparatuses for the following reasons.
In Japanese Patent Laid-Open No. 2002-257000, the piezoelectric actuators are used to open and close fuel injectors. Since the piezoelectric actuators are provided corresponding to a plurality of cylinders, even when one of the piezoelectric actuators breaks down, the cylinders corresponding to the other piezoelectric actuators can be used.
However, for example, when a plurality of optical elements in a projection optical system of an exposure apparatus are driven and a piezoelectric actuator corresponding to one optical element breaks down, it is difficult to maintain exposure accuracy by driving the optical elements corresponding to the other piezoelectric actuators.
In other words, when a short circuit is caused by migration in the piezoelectric actuator, it is determined that the exposure apparatus has broken down, and the piezoelectric actuator needs to be replaced. However, in order to replace the piezoelectric actuator, it is necessary to stop the operation of the exposure apparatus. This takes much time and much cost.
It is difficult for the current technology to completely prevent the piezoelectric actuator from migration. Accordingly, there is a demand for a driving device that can continuously drive the apparatus even when a short circuit is caused by migration.